Microwave-excited microplasma thruster with helium and hydrogen propellants

Microplasma thruster of electrothermal type has been investigated with feed or propellant gases of He and H2. The thruster consisted of an azimuthally symmetric microwave-excited microplasma source 1.5 mm in diameter and 10 mm long with a rod antenna on axis, and a converging-diverging micronozzle 1 mm long with a throat 0.2 mm in diameter. Surface wave-excited plasmas were established by 4.0-GHz microwaves at powers of ≤ 6 W, with the source pressure in the range 0.5–12 kPa at flow rates of 2–70 sccm. The microplasma generation, micronozzle flow, and thrust performance with He were numerically analyzed by using a two-dimensional fluid model, coupled with an electromagnetic model for microwaves interacting with plasmas in the source region. In experiments, the plasma electron density and gas temperature in the microplasma source were measured at around the top of the microwave antenna, or just upstream of the micronozzle inlet, by optical emission spectroscopy with a small amount of additive gases of H2 and N2. In the case of He propellant, the Stark broadening of H Balmer-β line and the vibronic spectrum of N2 2nd positive (0, 2) band indicated that the electron density was in the range (2-5)×1019m-3 and the gas or rotational temperature was in the range 600–700 K. The thrust performance was also measured by using a target-type microthrust stand, giving a thrust in the range 0.04–0.51 mN, a specific impulse in the range 150–270 s, and a thrust efficiency in the range 2%–12%. These experimental results were consistent with those of numerical analysis, depending on microwave power and gas flow rate. Similar plasma characteristics and thrust performance were obtained with H2 propellant, where the specific impulse of ≤ 450 s was more than 1.5 times higher than that with He, owing to a difference in mass between He and H2. A comparison with previous studies with Ar propellant [T. Takahashi et al., Phys. Plasmas 16, 083505 (2009)] indicated that in the presence as well as absence of plasma discharge, the specific impulse was enhanced by more than 3–5 times with light-mass propellants He and H2 as has been known for large-scale propulsion systems. Thus, it follows that in the microplasma thruster of electrothermal type, the high diffusivity and thermal conductivity of He and H2 in the microplasma source of high surface-to-volume ratios do not lead to a deterioration of the thrust performance, primarily owing to a more significant thermal energy gain due to elastic collisions between electrons and heavy particles in He and H2.